From nobody Sat Oct 25 20:21:22 2025 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) client-ip=208.118.235.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zohomail.com; spf=pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=fail(p=none dis=none) header.from=redhat.com Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 1520602281329126.83704223516634; Fri, 9 Mar 2018 05:31:21 -0800 (PST) Received: from localhost ([::1]:45298 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1euI7E-0000rV-Ho for importer@patchew.org; Fri, 09 Mar 2018 08:31:20 -0500 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47261) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1euI5Y-0008E7-Ug for qemu-devel@nongnu.org; Fri, 09 Mar 2018 08:29:37 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1euI5V-0003oz-Qe for qemu-devel@nongnu.org; Fri, 09 Mar 2018 08:29:36 -0500 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:37623 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1euI5V-0003o0-KY for qemu-devel@nongnu.org; Fri, 09 Mar 2018 08:29:33 -0500 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 78D044074CA7; Fri, 9 Mar 2018 13:29:29 +0000 (UTC) Received: from donizetti.redhat.com (ovpn-117-35.ams2.redhat.com [10.36.117.35]) by smtp.corp.redhat.com (Postfix) with ESMTP id EDA736353B; Fri, 9 Mar 2018 13:29:28 +0000 (UTC) From: Paolo Bonzini To: qemu-devel@nongnu.org Date: Fri, 9 Mar 2018 14:29:20 +0100 Message-Id: <20180309132922.24211-3-pbonzini@redhat.com> In-Reply-To: <20180309132922.24211-1-pbonzini@redhat.com> References: <20180309132922.24211-1-pbonzini@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.5]); Fri, 09 Mar 2018 13:29:29 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.5]); Fri, 09 Mar 2018 13:29:29 +0000 (UTC) for IP:'10.11.54.5' DOMAIN:'int-mx05.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'pbonzini@redhat.com' RCPT:'' X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 66.187.233.73 Subject: [Qemu-devel] [PATCH 2/4] rcu: make memory barriers more explicit X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Emilio G . Cota" Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Prepare for introducing smp_mb_placeholder() and smp_mb_global(). The new smp_mb() in synchronize_rcu() is not strictly necessary, since the first atomic_mb_set for rcu_gp_ctr provides the required ordering. However, synchronize_rcu is not performance critical, and it *will* be necessary to introduce a smp_mb_global before calling wait_for_readers(). Signed-off-by: Paolo Bonzini --- include/qemu/rcu.h | 15 +++++++++++++-- util/rcu.c | 12 +++++++++--- 2 files changed, 22 insertions(+), 5 deletions(-) diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h index f19413d649..625f09ac09 100644 --- a/include/qemu/rcu.h +++ b/include/qemu/rcu.h @@ -79,7 +79,10 @@ static inline void rcu_read_lock(void) } =20 ctr =3D atomic_read(&rcu_gp_ctr); - atomic_xchg(&p_rcu_reader->ctr, ctr); + atomic_set(&p_rcu_reader->ctr, ctr); + + /* Write p_rcu_reader->ctr before reading RCU-protected pointers. */ + smp_mb(); } =20 static inline void rcu_read_unlock(void) @@ -91,7 +94,15 @@ static inline void rcu_read_unlock(void) return; } =20 - atomic_xchg(&p_rcu_reader->ctr, 0); + /* Ensure that the critical section is seen to precede the + * store to p_rcu_reader->ctr. Together with the following + * smp_mb(), this ensures writes to p_rcu_reader->ctr + * are sequentially consistent. + */ + atomic_store_release(&p_rcu_reader->ctr, 0); + + /* Write p_rcu_reader->ctr before reading p_rcu_reader->waiting. */ + smp_mb(); if (unlikely(atomic_read(&p_rcu_reader->waiting))) { atomic_set(&p_rcu_reader->waiting, false); qemu_event_set(&rcu_gp_event); diff --git a/util/rcu.c b/util/rcu.c index f4d09c8304..7366dc50dd 100644 --- a/util/rcu.c +++ b/util/rcu.c @@ -92,8 +92,9 @@ static void wait_for_readers(void) atomic_set(&index->waiting, true); } =20 - /* Here, order the stores to index->waiting before the - * loads of index->ctr. + /* Here, order the stores to index->waiting before the loads of + * index->ctr. Pairs with smp_mb() in rcu_read_unlock(), + * ensuring that the loads of index->ctr are sequentially consiste= nt. */ smp_mb(); =20 @@ -142,8 +143,13 @@ static void wait_for_readers(void) void synchronize_rcu(void) { qemu_mutex_lock(&rcu_sync_lock); - qemu_mutex_lock(&rcu_registry_lock); =20 + /* Write RCU-protected pointers before reading p_rcu_reader->ctr. + * Pairs with smp_mb() in rcu_read_lock(). + */ + smp_mb(); + + qemu_mutex_lock(&rcu_registry_lock); if (!QLIST_EMPTY(®istry)) { /* In either case, the atomic_mb_set below blocks stores that free * old RCU-protected pointers. --=20 2.14.3